Glazes are recognized in the art of decorating glass, glass-ceramic, and ceramic bodies as being transparent glasses which are applied to provide decorative designs and finishes for the surfaces thereof to enhance the aesthetic appearance of the bodies, thereby promoting customer appeal. Glazes are conventionally applied to a surface in the form of a slurry or slip of finely-divided glass particles, commonly termed "frit", and, after drying the slip, this frit is fired to fuse the particles to form a well-bonded, continuous glassy coating on the surface of the body.
Commercially marketed frits have customarily contained high levels of lead oxide (PbO) and, less often, substantial amounts of cadmium oxide (CdO). Those oxides display two properties which make them particularly desirable to use in glazing frits. First, they "soften" the glass; i.e., they lower the melting point of the glass such that it can be fused to flow along the surface of the body to be glazed at a temperature which is sufficiently low to preclude thermal deformation of the body. Second, they raise the refractive index of the glass, thereby heightening the gloss exhibited by the glaze. CdO has also been utilized as a colorant in certain frits. Unfortunately, however, both CdO and PbO are highly toxic such that stringent regulations have been applied by the Food and Drug Administration (FDA) with respect to their release when compositions including those compounds come into contact with food.
Thus, the FDA has established maximum limits for the release of lead from glazed surfaces. In the prescribed test a glazed surface is contacted with an aqueous 4% acetic acid solution at room temperature for 24 hours. A sample of the acetic acid solution is thereafter tested for absorbance in an atomic absorption spectroscope and the observed value converted to a metal concentration value on a standard curve, the lead content being reported in parts per million (ppm). That lead value is based upon the inside volume of a hollow article having a glazed or decorated surface and filled to a specified level with the acetic acid solution for the test. A similar test has also been devised for use on exterior surfaces of a vessel or dish.
U.S. Pat. No. 4,582,760 (Beall et al.) discloses glazes designed for application to glass-ceramic articles wherein potassium fluorrichterite crystals comprise the predominant crystal phase therein or wherein potassium fluorrichterite plus cristobalite in amounts in excess of 10% by volume, commonly about 10-20%, constitute essentially the sole crystal phases. The former glass-ceramic articles are described in U.S. Pat. No. 4,467,039 (Beall et al.) and consist essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 50-70 Na.sub.2 O 2-9 CaO 4-15 K.sub.2 O 2-12 MgO 8-25 Li.sub.2 O 0-3 F 3-8 Al.sub.2 O.sub.3 0-7. ______________________________________
The latter glass-ceramic articles are reported in U.S. Pat. No. 4,608,348 (Beall et al.) and consist essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 65-69 Na.sub.2 O 1.5-3.5 Al.sub.2 O.sub.3 0.75-3.5 K.sub.2 O 4.2-6 MgO 13.5-17.5 BaO 0-2.5 CaO 3-4.8 P.sub.2 O.sub.5 0-2.5 Li.sub.2 O 0.5-2 F 3.3-5.5. ______________________________________
The glazes disclosed in U.S. Pat. No. 4,582,760 demonstrated a lead release not exceeding 1 ppm when subjected to the FDA test procedure and consisted essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 46-50 Na.sub.2 O 2-3.5 SrO 2-6 Al.sub.2 O.sub.3 5-8 K.sub.2 O 1-2 B.sub.2 O.sub.3 9-11 ZrO.sub.2 0.3-1 CaO 1-4 PbO 20-25. Li.sub.2 O 1-2 ZnO 2-6 ______________________________________
That release of lead is far below the maximum limit permitted by the FDA.
Nevertheless, because of continued public pressure for the complete elimination of lead and cadmium from materials coming into contact with food, research has been constant to develop lead- and cadmium-free glasses operable as frits for glazing glass, glass-ceramic, and ceramic articles. It will be recognized that the removal of PbO and/or CdO from the glass composition has required the substitution of other ingredients to confer the necessary chemical and physical properties to the frits. That is, the frits must demonstrate the following attributes:
(1) the frit must manifest good glass stability, i.e., the frit should not devitrify during firing to coat the surface of an article;
(2) the frit must exhibit excellent resistance to attack by acids and bases because corrosion of the glaze can lead to loss of gloss, the generation of haze and/or iridescence, the development of porosity, or other defects deleterious to the appearance and/or physical character of the glaze;
(3) the firing or maturing temperature of the frit, i.e., the temperature at which the frit displays enough flow to produce a smooth homogeneous coating, ought to be sufficiently low to inhibit thermal deformation of the body being coated; and
(4) the linear coefficient of thermal expansion of the frit must be compatible with that of the article being coated to preclude crazing and/or spalling, with the preferred frits having a linear coefficient of thermal expansion somewhat lower than that of the article being coated, thereby placing the matured coating under compressive stress when the coated body is cooled to room temperature.
The linear coefficients of thermal expansion of the glazes set forth in U.S. Pat. No. 4,582,760, supra, ranged between about 60-70.times.10.sup.-7 /.degree.C. over the temperature range 0.degree.-300.degree. C., thereby assuring the development of surface compression when applied to the glass-ceramic bodies of U.S. Pat. No. 4,467,039 and U.S. Pat. No. 4,608,348, supra, those bodies demonstrating linear coefficients of thermal expansion (0.degree.-30.degree. C.) in the region of about 80-100.times.10.sup.-7 /.degree.C. for the articles of U.S. Pat. No. 4,467,039 and about 100-140.times.10.sup.-7 /.degree.C. for the articles of U.S. Pat. No. 4,608,348.
It will be recognized that, where the glaze-coated bodies are intended for use in food preparation and/or service applications, the glazes must resist attack by acids and bases present in foods and by detergents such as are employed in commercial dishwashers.
Corning Incorporated, Corning, N.Y., has marketed a glass-ceramic dinnerware as Corning Code 0308 under the trademark SUPREMA.RTM. which is encompassed within U.S. Pat. No. 4,608,348, supra, in that it contains potassium fluorrichterite and cristobalite as the predominant crystal phases and has the following approximate base composition, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 66.8 K.sub.2 O 4.75 Sb.sub.2 O.sub.3 0.23 Al.sub.2 O.sub.3 1.7 Na.sub.2 O 3.3 NiO 0.014 MgO 14.5 Li.sub.2 O 0.8 Co.sub.3 O.sub.4 0.0015 CaO 4.4 P.sub.2 O.sub.5 1.15 Fe.sub.2 O.sub.3 0.02. BaO 0.21 F 3.74 ______________________________________
A color package consisting, in weight percent, of
______________________________________ 0.065-0.16 Fe.sub.2 O.sub.3 0.055-0.16 NiO 0.0015-0.0029 Se 0-0.0014 Co.sub.3 O.sub.4 ______________________________________
has been added to the base composition for SUPREMA.RTM. dinnerware to impart a beige tint thereto, that addition being disclosed in U.S. Pat. No. 5,070,043 (Amundson, Jr. et al.). The crystal microstructure and the chemical and physical properties of the ware are not altered by the addition. The product, marketed as Corning Code 0309, has the following approximate composition, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 66.1 K.sub.2 O 4.75 Sb.sub.2 O.sub.3 0.07 Al.sub.2 O.sub.3 1.67 Na.sub.2 O 3.15 NiO 0.085 MgO 14.11 Li.sub.2 O 0.78 Co.sub.3 O.sub.4 &lt;0.0005 CaO 4.63 P.sub.2 O.sub.5 1.15 Fe.sub.2 O.sub.3 0.07 BaO 0.22 F 3.3 Se 0.0018. ______________________________________
The primary objective of the present invention was to devise lead-free and cadmium-free glazes operable for use with glass-ceramic bodies having compositions within the disclosure of U.S. Pat. No. 4,608,348 and U.S. Pat. No. 5,070,043. Stated in another way, the goal of the invention was to find a replacement for the glazes described in U.S. Pat. No. 4,582,760; that is, glazes exhibiting chemical and physical properties similar to those demonstrated by the patented glazes, but having no lead or cadmium content in their compositions.
A specific objective of the present invention was to develop a lead-free and cadmium-free glaze suitable for application to ware marketed under the trademark SUPREMA.RTM..